Wish to understand why non-stock vehicle alignment is an issue

Question

I just replaced all of my suspension components (Shocks, Springs, Hangers, Shackles) on my Ford Excursion with that of an F350 + a few modifications to make the rake appear level. I also have slightly larger than stock (33"x11") tires on 18" rims.

I would like the alignment checked. The truck drives/steers perfectly. But I believe my camber / toe could still be off from the suspension change. I don't want to ruin my new tires.

My question: Why are shops telling me they can't align my vehicle "because factory specs no longer apply"? What does that even mean? Specs for what? My understanding is that for Toe and Camber, you simply want none / flat on the pavement. Are "factory specs" just for Caster? And I guess the question would be, ok who cares - so set it to factory then? I don't see how an extra 2" of suspension would change factory spec for toe/camber, maybe slightly for Caster? Furthermore, I was told (by a shop who refused) to seek out an old-school tape measure, string and marker style alignment shop. Why is that? Why can't someone use a computerized alignment machine? Pretty shocked / lost here. I've had non-stock trucks all my life and have never run into this previously, until attempting to get it done here in Ontario Canada.

If I have to go looking all over for a competent alignment shops, i'd like to be more educated on this so I can ask the right question. The non-stock = we can't policy seems like pure cop-out idiocy or blind corporate policy. Looking for some education. Thanks!

Answer 1

Factory specs assume factory parts. Each modification you have made changes the angles at which the suspension operates so those angles no longer apply. This is especially true on truck lifts where a large amount of the suspension is replaced.

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Here is a nice tire rack information write up (or read below): https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=4

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As for the shop, it is not a matter of competency, just a matter of experience and liability. The shop in question could very well align the car to factory specs. If the car handles poorly or then exhibits undesirable behavior, then what? They can fix it, but how does the technician know what angles to set? They are not a trained engineer working under an engineering team who has worked all these things out in advance.

I have met plenty of extremely experienced technicians (30+ years of experience) who would take your truck and set the various adjustment points completely off from factory specs and let you go on your way. The truck would ride fine and the tires would wear reasonably well.

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To keep it simple but answer your comments on toe/camber/caster:

Toe out is always bad, it will make the steering feel loose and the car can "wander" while the steering wheel is straight. Neutral toe will cause uneven wear because of turning and the way the geometry of the suspension puts pressure on a certain section of the tire. Toe in helps prevent some of that but does not fully prevent the tire from wearing unevenly as it will now wear unevenly when driving straight. Factory specs on most cars keep toe in under a few degrees but you will often see racing vehicles with more aggressive toe settings because it helps with turn in.

Positive camber is only helpful on crowned roads, otherwise it produces weird unpredictable steering and handling on flat roads and while turning. Neutral camber is okay but will produce premature tire wear when turning and become more negative if a heavy load is put on the car. Negative camber can be desirable up until the point it reduces the tires contact patch. I think the typical sedan has something like a degree of negative camber.

Caster is largely set by the manufacturer and they design the other steering angles to compensate against any negative effects. It shouldn't be adjusted and the factory angle should be maintained as caster being significantly off indicates damage somewhere in the suspension.